This invention relates to the measurement of the optical characteristics of deposited thin films. More specifically, the present invention provides a continuous measurement of the optical transmissivity of a film as it is being deposited on a substrate. Such a measurement is useful in a variety of situations.
As an example, bubble canopies and windshields of aircraft are often coated with an electrically conductive transparent film which allows the canopy or windshield to be heated, and it is desirable to continuously measure the optical transmission through the windshield during the coating process so as to ensure adequate clarity of the coating. Also, since the optical transmission through a deposited thin film is a function of its thickness, the present invention can be used as an indirect measurement of film thickness. For films having an optical thickness much less than one-quarter wavelength of the light passing through it, the optical transmission is inversely related to its thickness. For thicker films, the thickness also affects transmission due to the interference pheonomenon caused by the internal reflections present in the film.
One common way of depositing a thin film on a substrate is known as "sputtering". This process is carried out in a partial vacuum under the influence of an electric field. A continuous measurement of optical transmission through the part being coated is difficult because of the problems involved in appropriately locating the optical measurement components to avoid shadowing.
Even those prior art instruments which entail the use of auxiliary substrates to monitor the process include optical systems which have disadvantages. See for example, U.S. Pat. Nos. 2,771,055 and 3,063,867.
The sputtering process mentioned above involves the use of a cathode fitted with or comprised of the target material to be sputtered. The cathode is kept at a high negative potential of several kilovolts--up to 5 kV--for diode and triode process configurations, or, at several hundreds of Volts--commonly 300 to 600 Volts--for magnetically enhanced cathode types. This latter type of cathode is commonly used in planarmagnetron and post-magnetron configurations. Radio frequency excitation of the sputtering cathode is also common.
The chamber atmosphere is commonly argon for simple non-reactive sputtering of materials directly onto a substrate. However, a mixture of gases may be used when reactive sputtering is employed. In this latter case, a chemical compound created by a reaction of the target material with the sputter gases is deposited onto the substrate. The pressure of the sputter gases in the vacuum chamber are commonly held at 10 to 100 micron for the non-magnetically enhanced sputtering methods and at 1 to 10 micron for magnetically enhanced methods.
Whatever method is used, the electric field causes ionization of the sputter gases creating a glow discharge. This glow discharge is commonly called the plasma. It exists in the form of a highly energetic cloud of electrons and positively charged sputter gas ions. The ions are attracted to the target and bombard the target surface, ejecting atoms of the target material. These atoms leave the target at high velocity in a distribution pattern determined by the sputtering method and cathode used, and in some cases, the geometry of the cathode and the surrounding apparatus. The substrate to be coated is placed so that these atoms strike the substrate surface and adhere; thus forming a film of the target material (or a compound of the target material if reactive sputter gases are used).
An optical measurement system set up to monitor or control the sputtering process must avoid having parts in a position too near to the object being coated and/or the plasma zone such that it would distort the distribution pattern of the atoms arriving at the object; and thereby "shadowing" the object. This shadowing can cause a nonuniform coating or distort an otherwise intended non-uniform global coating pattern.
Even if an optical system can be set up which avoids shadows, a different optical system must be devised for each application, which of course is inconvenient and expensive. Accordingly, the herein disclosed monitor avoids the difficulties arising because of the placement of the optical components in an optical transmission measuring system for deposited coatings by providing a self contained measuring system which measures the optical transmissivity of a monitor substrate wherein the shadowing effect is controlled.